Recently, an information communication system has been developed in which communications are conducted in a contactless manner between a master station, or a reader/writer and slave stations, or IC cards. The practical use of this communication system is now under study in various fields. For example, if the system was applied to a ticket gate of a railroad station, IC cards would be used as commuter tickets and a reader/writer would be set up in an automatic ticket gate, whereby the opening and closing of the gate is controlled in accordance with the information communication conducted between the IC cards and the reader/writer. This eliminates users having to take commuter tickets out of card cases. As such, the system is practically useful and it can be expected that the system will be applied in wide range of fields in the future.
In the above information communication system, there may be a case where, for example, a user carries a plurality of commuter tickets (slave stations), they respond to a request from the master station by transmitting a plurality of response signals simultaneously. The system, however, has a problem that the master station cannot appropriately recognize overlapping response signals from the slave stations. This problem is especially serious when the master station attempts, at the start of the communication, to acquire the identifiers (hereafter abbreviated as “IDs”) of the slave stations. Once having acquired the IDs of the slave stations, the master station can separately transmit or receive information to/from the slave stations by adding the IDs to signals.
As one method for avoiding the overlapping of response signals from slave stations, use of time slots is currently studied. In this method, a time period during which the slave stations are allowed to transmit their response signals is divided into a plurality of time slots having a certain time length. Each slave station then selects one time slot at random and transmits a response signal during the selected time slot.
The following describes the above-mentioned method using time slots. FIG. 1 is a timing chart for explaining a case where a master station 901 conducts information communications with each of a plurality of slave stations (in this example, a slave station A 902, a slave station B 903, a slave station C 904, and a slave station D 905).
When receiving the first ID request signal 911 transmitted from the master station 901, each slave station transmits an ID response signal with timing corresponding to a time slot number which is determined by the slave station at random. In the illustrated example, the slave station A 902 selects a time slot with the slot number 1 and transmits an ID response signal 912 with timing corresponding to the selected time slot number 1. The remaining slave stations B 903 to D 905 select a time slot with the slot number 2 and transmit their ID response signals with timing corresponding to the selected time slot number 2.
In this case, the slave station 901 can recognize the ID response signal 912 transmitted from the slave station A 902, because only the ID response signal 912 is transmitted during the time slot 1 without overlapping. However, the master station 901 cannot recognize any of the ID response signals 913, 914, and 915 that are transmitted during the time slot 2 since they overlap with one another. As a result of this, the master station 901 needs to transmit an ID request signal 916 for the second time to each of the slave stations B 903 to D 905 to acquire the IDs.
Upon receiving the second ID request signal 916, each slave station transmits the ID response signal for the second time with timing corresponding to the time slot number selected at random. Note that the second ID request signal 916 is attached with information indicating that the slave station A 902 need not transmit the ID response signal to the second ID request signal 916, because the master station has already acquired the ID of the slave station A 902. In this way, by retransmitting the ID request signal, the master station 901 can finally acquire the IDs of all the slave stations.
As another method for avoiding response signals transmitted from slave stations from overlapping with one another, the number of time slots may be increased. For example, if ten time slots instead of four shown in FIG. 1 are provided, a probability that a plurality of slave stations select the same time slot will be decreased.
However, both methods (i.e., the method of allowing the master station 901 to repeat transmitting the ID request signal and the method of increasing the time slot numbers) have a problem that the whole processing time tends to be increased. For instance, in the case where IC cards are used as commuter tickets conducting information communication with a reader/writer set up in a ticket gate, all processes including a process for writing data into each slave station have to be completed in a predetermined time period during which the slave stations are in an area in which they can communicate with the master station (hereinafter, this area is referred to as “service area”). In these circumstances, increase in the processing time may cause a problem that the whole process does not complete in the predetermined time period. This is a serious problem from a practical point of view.
In view of the above-described problems, it is an object of the present invention to provide an information communication system that can reduce the whole processing time in the information communications conducted between a master station and a plurality of slave stations, a contactless IC card used as a slave station in the information communication system, and an IC chip.